a) Field of the Invention
The present invention is directed to a measuring method for ophthalmology which uses short-coherence interferometry to measure the positions of reference points on the basis of which the three-dimensional structure of all refracting and reflecting intraocular interfaces and surfaces can be depicted, for example, by means of spline surfaces or polygon surfaces.
b) Description of the Related Art
For this purpose, measurement beams are radiated into the eye simultaneously through a number of pupil points, and the depth positions at which these measurement beams pierce or are reflected at intraocular interfaces and surfaces are determined by spectral short-coherence interferometry (also known as Fourier domain OCT). Based on these piercing points and reflection points, the shape of these surfaces can be calculated numerically and used for depicting the structure of the eye in three dimensions by means of known computer graphics techniques such as spline surfaces or polygon surfaces.
For this purpose, the object to be measured is placed in one arm of a two-beam interferometer. The spectrum of the light bundle coming from the measured object is superimposed with the reference bundle at the interferometer output and the resulting spectral intensity is evaluated by spectrometry.
Roughly over the last 15 years, various experimental ray tracing methods have been developed which are suitable for measuring the characteristics of optical systems. For example, Navarro and Losada (“Aberrations and relative efficiency of light pencils in the living human eye”, Optometry and Vision Science 74(7), 540-547, 1997) illuminate the pupil of the eye with parallel laser beams and determine the imaging characteristics of the eye from the position of the remitted beam on the photocathode of a CCD camera which is conjugated to the retina with respect to imaging. The application of this method for measuring the imaging characteristics of optics is described by Navarro and Moreno-Barriuso (“Laser ray-tracing method for optical testing”, Optics Letters 24(14), 951-953, 1999). However, in this ray tracing method the cumulative total effect of all refracting surfaces is measured. The determination of the shape of individual interfaces and, therefore, a causal relationship between aberrations of the eye and the causative interfaces can only be approximated.
At present, ray tracing is known chiefly as a computer graphics method for depicting realistic illumination conditions in three-dimensional reproduction (rendering) of computer images. However, ray tracing, along with wavefront tracing, is also a numerical process for calculating the light propagation in inhomogeneous media. Whereas ray tracing, which is based on the laws of refraction, is already used in optics for optical design, the application of wavefront tracing, which is based on Huygens' principle, is known rather in seismology for calculating the propagation of seismic waves.
In optical coherence tomography, images are generated from short-coherence interferometric measurement data of so-called A-scans. These A-scans determine the object structure as a distribution of light-remitting points along the measurement beams in the depth of the object. The light remitted by the object is correlated with a reference beam in the short-coherence interferometer by displacing the reference mirror. On the other hand, according to the known rules of Fourier domain OCT, the A-scan data are calculated by Fourier transformation of the intensity spectra of the light that is remitted by the object and superimposed with a reference beam (Bouma, B. E., Tearney, G. J., Handbook of Optical Coherence Tomography”, Marcel Dekker Verlag, New York, chapter 12, 2002).
For purposes of ray tracing in the eye it is necessary to illuminate the pupil of the eye at a plurality of distributed pupil points and to evaluate the measurement beams in a corresponding manner. In known time domain OCT, a pupil scan in which the measurement beam is guided consecutively to the various pupil points by a scanning unit is required for this purpose. The intensity spectrum of the A-scan would have to be measured at every point. This is time-consuming and can lead to motion artifacts.
For this purpose, a superluminescent diode, a light-emitting diode (LED), a mode-lock laser, an ASE (Amplified Spontaneous Emission) fiber light source, a photonic crystal fiber light source, a thermal light source (incandescent lamp), or a plasma light source (arc lamp), for example, can be used as a short-coherence illumination source.
Similar optical Fourier OCT methods which are known from the art will first be discussed in the following.
Publications by A. F. Fercher et al. (“Measurement of optical distances by optical spectrum modulation”, Proc. SPIE Vol. 2083, 263-267, 1993) describe the optical Fourier OCT method in general and also the specific determination of the coherence function of the light reflected by the eye through inverse Fourier transformation of the spectral intensity distribution I(ω) (“In Vivo Optical Coherence Tomography in Ophthalmology”, Bellingham, W. A., SPIE, pp. 355-370, ISBN 0-8194-1379-8, 1993).
The use of Fourier transform methods specifically for measuring intraocular distances along an individual beam through the pupil was described by A. F. Fercher et al. (“Measurement of Intraocular Distances by Backscattering Spectral Interferometry”, Opt. Commun. 117, 43-48, 1995) and used by G. Häusler and M. W. Lindner for producing OCT images (“Coherence RADAR” and “spectral RADAR”—New Tools for dermatological diagnosis”, J. Biomed. Opt. 3(1), 21-31, 1998).
DE 43 09 056 A1 describes a method for determining the distance and scattering intensity of scattering points in which the distance and the local scattering intensity are determined by Fourier transformation of the spectrum according to wavelength.
A method in which three-dimensional images of the retina can be synthesized from en-face OCT recordings was described by A. G. Podoleanu, J. A. Rogers, D. A. Jackson, and S. Dunne (“Three dimensional OCT Images from retina and skin”, Opt. Express 7, pp. 292-298, 2000).
A parallel OCT method which likewise uses a step reference mirror is described in U.S. Pat. No. 6,268,921 B1. The step reference mirror is used to implement the depth scan in time domain OCT. Accordingly, the step sizes are also appreciably greater than λ/8. Further, the steps are distributed over the entire surface in a staircase shape rather than with periodically recurring total heights. The phase shifter used in this solution acts identically on the entire reference arm or measurement arm. These differences are a natural result of the different problem to be solved as stated therein.
A similar method which is based on piezoelectric phase shifting phase measurement is disclosed in U.S. Pat. No. 6,377,349 B1. In this solution, the displacement of the reference mirror is effected by means of piezo electricity. However, this displacement, the required additional exposures and the repeated readout of the photodetector array require time which leads to motion artifacts in in vivo objects like the eye.
A conventional OCT method for determining the dimensions of the anterior segments of the eye using a slit lamp and a hand-held device was described by S. Radhakrishnan et al. (“Real time optical coherence tomography of the anterior segment using hand-held and slit-lamp adapted systems”, Proc. SPIE 4619, 227-229, 2002). The device which is based on time domain OCT works very fast and delivers eight images per second. For example, the eight images per second can be distributed equidistantly on the entire pupil for a three-dimensional depiction of the anterior eye structure; about one second would then be required for data recording. In contrast, the method upon which the present application is based can register the required data within milliseconds.